The present invention relates to the palladium alloys and use of such palladium alloys for the preparation of firmly fixed and removable dentures which are admixable with dental ceramics.
Dentures are predominantly prepared from corrosion resistant and biocompatible noble metal alloys according to the so-called wax melt process, whereby the cast object is often subsequently admixed (blended) with dental ceramics in order to obtain an appearance corresponding to natural teeth. In addition to a suitable strength, these alloys must possess special properties required for dental ceramics, e.g. expansion coefficient, melting interval, and adhesion between the ceramic and the alloy.
Alloys having a high gold content, e.g. alloys described in German patents 11 83 247 and 15 33 233, are particularly suitable for this purpose Because of the high price of gold, recent efforts have been made to find cheaper alternatives for alloys having a high gold content. In the field of noble metals, palladium seems to be a suitable substitute because palladium is relatively cheaper than gold, possesses a significantly lower density than gold, and shows a corrosion resistance and stability in the mouth similar to that of gold.
The hitherto developed fired alloys, which contain palladium as the main component, can be roughly divided into three groups:
The first group consists of gold-palladium alloys which are generally free of silver or have a very low silver content. Typical representatives of such alloys are described in German patents 34 06 712 and 29 44 755. However, these alloys are still relatively expensive due to the high gold content thereof.
The second group consists of palladium based alloys which are generally free of silver, e.g. alloys disclosed in German patents 33 16 595, 33 04 183, 35 22 523 and 32 47 398. The main alloy components of these alloys are copper and gallium. Further alloy components often used in these systems are tin, indium and cobalt As compared to fired alloys having a high gold content, the latter alloys are more difficult to be processed and are more sensitive to failures occurring during processing. These alloys are not easily solderable and take up carbon in the molten state, and for this reason they can be melted only in ceramic crucibles in order to avoid bubble formation in the fired ceramic mass. Moreover, all these alloys form oxides having a color from dark to black during the firing of the ceramic. Thus dark edges are formed in the marginal region of the blended mixture which adversely affect the aesthetic appearance of the dentures.
In U.S. Pat. No. 4,539,176, palladium alloys free of silver are disclosed which contain, in addition to palladium, 10- 40% by weight gold, 3-8% by weight gallium, 0.5-10% by weight indium and/or tin, and 0.1-1.5% by weight ruthenium, iridium or rhenium. However, these alloys are unsuitable for the desired purposes.
The third group consists of palladium-silver alloys. The processing properties of these alloys are between those of alloys having a high gold content and palladium alloys free of silver. The typical composition of these alloys is disclosed in "Review of dental noble metal alloys and dental non-noble alloys in the German Federal Republic" (Ubersicht uber die Dental-Edelmetallegierungen und Dental-Nichtedelmetallegierungen in der Bundesrepublik Deutschland), published by the "Research Institute of Dental Care" (Forschungsinstitut fur die zahnarztliche Versorgung), Jul. 1, 1986, pages 31-32. In addition to palladium and silver, these alloys predominantly contain tin, indium and zinc, and occasionally copper or gallium as further alloy components. The oxide color of these alloys is generally significantly lighter than that of silver-free palladium alloys. However, it is a drawback that these alloys give a yellow or yellowish-green discoloration to the ceramic blend during the firing process. The reason for this phenomenon is that silver migrates by diffusion or through the vapor phase into the ceramic. Several attempts were made to eliminate or at least reduce this tendency to discoloration of palladium-silver alloys by corresponding alloying conceptions.
In order to achieve this object, the addition of titanium in an order of magnitude of 0.1-0.5% is suggested by German patent 25 23 971, and the addition of 0.1-1% of silicon is suggested by U.S. Pat. No. 4,350,526. However, both of these solutions are accompanied by serious disadvantages from the point of view of dental processing properties. Both elements cause the embedding to adhere strongly to the casted objects and this makes the removal and finishing more difficult and slower.
Palladium forms with silicon intermetallic phases so that a strong embrittlement of the alloy and broken casting may occur. Moreover, both titanium and silicon are highly reactive to oxygen in the air, and for this reason the amount of these elements in the melt decreases relatively rapidly and this causes a reduction of the desired effect. This is particularly relevant when aged scrap material (cast channels, casting funnels) are applied.
According to German patent 39 05 987, palladium-silver alloys are made insensitive towards discoloration by adding germanium together with gallium and/or cobalt in a defined weight ratio. However, under unfavorable geometrical conditions and at the edges of the blend, ceramics sensitive towards discoloration may show signs of discoloration. Moreover, the market shows fundamental reservations against silver-containing palladium alloys even if the tendency to discoloration is reduced or completely eliminated.
In German patent 31 46 794, noble metal alloys suitable for firing of dental porcelain are disclosed. The alloys contain 0.05-1% by weight of one or more of the elements tantalum, tungsten and/or yttrium, in addition to 20-85% by weight palladium, 0-55% by weight of gold, 0-40% by weight of silver, 1-15% by weight tin and/or indium, and 0.1-3% by weight gallium, in order to improve the breaking elongation and casting brittleness. The disadvantage of tantalum and yttrium is that these metals are oxidized very easily and this results in slag formation in the crucible Tungsten forms a volatile oxide and this is disturbing during melting because of the formation of smoke and pores in the texture of the cast structure.